1. Field of the Invention
The invention relates to the general field of infrared detectors, more particularly to the use of tuned cavities.
2. Description of the Prior Art
FIG. 1 is a schematic diagram of a Fabry Perot interferometer. Its basic components are a pair of transparent plates 5 and 6, spaced a few millimeters apart, whose inner surfaces have been coated with a thin film of metal which reflects almost all of a light beam that may be incident upon it but does transmit a small proportion thereof. Typically about 90% of the light is reflected and about 10% is transmitted, absorption being negligible.
Thus about 10% of light beam 1, entering the system at other than normal incidence, will enter the inter-plate cavity where it will be successively transmitted and reflected at both of the inner surfaces. It is readily seen that the transmitted beams that emerge on the far side of plate 6 (designated 2,3,4 in FIG. 1) are all coherent relative to each other and adjoining beams differ in phase from one another by the same amount. If such beams are caused to converge by means of a lens (not shown, but located to the right of plate 6) they will form interference fringes, dark where they were out of phase, light where they reinforced one another.
Interferometers of the type illustrated in FIG. 1 can have very high resolving power but they are very sensitive to a number of factors. These include the reflectivity and optical finish of the surfaces involved and establishment and maintenance of parallelism. Additionally, small variations in optical path length within the cavity that may result from temporary or permanent inhomogeneities of the material contained in the cavity, must be avoided.
A number of these problems are removed or mitigated if the plates of the interferometer are concave, as illustrated in FIG. 2. Light beam 21 enters the cavity by passing through partially reflecting surface 25. After making four passes back and forth between the two concave surfaces it has reached region 22, which is approximately its point of entry. After four more passes it is back at 22 again, and so on. This arrangement of two concave plates need not be perfectly aligned to still function. Nor is it as sensitive to external vibration as the parallel plate version.
An optically equivalent version of the double concave structure can be constructed by using a single concave plate in conjunction with a planar mirror. A virtual image of the concave surface is created by the planar mirror so a light beam will end up in the same spot within the cavity after every eight passes. An interferometer of this type forms the subject matter of the present invention and may be looked at as a resonant cavity in the microwave sense.
We are not aware of any prior art relative to infrared radiation detectors that is based on the use of a resonant cavity. Cole (U.S. Pat. No. 5,286,976 FEB 1994) teaches a detector that is partly transparent to infrared and is backed up by a mirror so it receives additional input from the reflected beam, making it, in effect, a two pass cavity. This is not, however, a resonant cavity and Cole's detector is sensitive over a wide range of infrared wavelengths. The present invention, by contrast, is based on an eight pass resonant cavity and, for a given geometry, is sensitive to only a narrow band of wavelengths.
Liddiard (U.S. Pat. No. 4,574,263 Mar 1986) also provides a mirror to increase the sensitivity of the detector, but notes that said mirror is optional and not key to the invention (which it is in the present invention).